The present invention relates to a hydraulic chain tensioner having a piston longitudinally movable in a fluid chamber and more particularly, to a porous air vent to permit the escape of air from inside the fluid chamber in such hydraulic chain tensioner.
Hydraulic tensioners are typically used as a control device for a chain drive in an automobile engine timing system. The tension in the chain can vary greatly due to the wide variation in the temperature and the linear expansion among the various parts of the engine. Moreover, wear to the chain components during prolonged use can produce a decrease in the tension of the chain. A hydraulic tensioner is used to take up the slack in the chain or belt that connects the camshafts to the crankshaft of the engine timing system.
A typical hydraulic tensioner is comprised of a housing having a bore, a fluid chamber defined by the bore, and a piston biased in a protruding direction from the bore by a spring. A check valve is also included in the hydraulic tensioner to permit fluid flow from a reservoir or oil supply passage into the fluid chamber, while preventing back flow in the reverse direction. The force of the chain against the piston in an inward direction is balanced by the resistance force of the fluid and the force of the spring in an outward direction.
A potential problem with hydraulic tensioners is that the fluid pressure inside of the hydraulic bore may change due to the introduction of air into the fluid. If air, a compressible fluid, becomes trapped inside the fluid chamber, the tensioner will not be fully effective in providing a load against the chain. Therefore, it is desirable to purge any air from the fluid chamber of the tensioner.
One method of venting air is described in Mittermeier, U.S. Pat. No. 4,504,251. Mittermeier discloses a tensioner with a hollow piston longitudinally guided in a hydraulic housing closed off by a plug provided with an external thread. The plug is pressed into a smooth central bore of the piston, and the thread grooves communicate with the atmosphere through radial channels, which permits venting of the housing. Another venting possibility exists through an excess pressure valve built into an ante-chamber arranged parallel to the piston. This excess pressure valve opens when the oil pressure exceeds a predetermined maximum in order to permit the oil and air mixture that accumulates in the upper part of the ante-chamber to exit into the atmosphere. The mixture, which flows continuously at a low flowing force, is guided toward the chain by a spraying pipe and used for the lubrication of the chain.
Another method for venting air disclosed in Hunter et al., U.S. Pat. No. 5,346,436, which is incorporated herein by reference, uses an air vent in the piston of the tensioner. The tensioner housing has a fluid filled chamber, a hollow plunger with an aperture in its upper end slidably received within the chamber and biased in a protruding direction by a spring, and a check valve provided between the chamber and the source of fluid pressure. The air vent comprises a disk biased by the spring against the inside of the upper end of the plunger. The vent disk has a channel formed between the atmosphere and the fluid reservoir. The channel has a first end at the outside of the disk and a second end at a position that connects the atmosphere and the chamber through the aperture in the plunger. To control fluid flow from the chamber and to restrict or minimize the quantity of air entering the chamber, the channel has a circuitous path from the first end to the second end. In addition, the aperture may contain a restricting member to further restrict the flow of fluid from the chamber and a rack and ratchet assembly to provide a mechanical no-return function.
Another form of venting is shown in U.S. Pat. No. 5,718,650, to Smith et al., which is incorporated herein by references. That vent allows the escape or venting of air through a vent in the upper end of the piston or in the oil supply inlet. The vent is porous and formed of sintered powder metal. The porous nature of the sintered powdered metal allows air to pass through quickly, but due to the higher viscosity of oil, limits the flow rate of oil. High air flow is desired through the vent to purge the air quickly, while restricting outward oil flow to maintain good control of the timing drive system. The air flow can be increased by altering the shape of the vent.
In FIG. 4, a hydraulic tensioner with a venting system of the prior art includes is illustrated. The tensioner 200 includes a piston 202 received in a housing 204. A fluid chamber is formed between the housing and the interior of the hollow piston. An air vent piece 206 is biased upward by a piston spring 208. An air vent sleeve 210 is interposed between the piston and the vent piece. Air vents between through the clearance 212 between the sleeve and the piece 206.